Process for concentrating aqueous formic acid



Patented Mar. 17, 1936 UNITED STATES PROCESS FOR CONCENTRATING AQUEOUSFORMIC ACID Otto Dragendorif, Oestrich, Germany, assignor to the firmRudolph Koepp & C0. Chemische Fabrik A. G., Oestrich, Germany, acorporation of Germany No Drawing. Application October 18, 1934, SerialNo. 748,961. In Germany September 30, 1933 10 Claims.

This invention relates to a process for concentrating formic acid.

It is known that anhydrous formic acid cannot be obtained from itsaqueous solutions by simple 5 distillation, since a 77% formic acidconstitutes a constant boiling mixture. Various processes have alreadybeen proposed for concentrating formic acid in another manner, bycombination with suitable substances and separation from the water or byazeotropic distillation. None of these processes have shown absolutelysatisfactory results, in part because they are attended with losses, inpart because the operation had to be effected with large quantities ofsolid salts, in part, however, also because they do not attain thedesired object.

It has long been known (of. for example Gardner, Berichte, volume 23,page 1587, (1890)) that organic acids, including formic acid, arecapable of forming acid or neutral salts, which can be relatively easilydecomposed, with organic bases, such as quinoline, diethylaniline,pyridine, picoline and others.

It has now been found according to this invention that formic acid canbe concentrated in a simple and efficient manner, by combining it withan organic base to form an acid or neutral salt and first azeotropicallydistilling off from this salt the water with the aid of an auxiliaryliquid and then azeotropically distilling off the formic acid withdecomposition of the salt. It

was found that the acid, and in part even the neutral, salts of formicacid with organic bases can be decomposed into their components withoutdifiiculty and even at low temperatures by azeotropic distillation. If,for example, a 35% formic acid is mixed with so much diethylaniline,that 1 mol. of base is employed per 2 mols of concentrated acid, tolueneis added and the mixture is distilled, the azeotropic mixturewatertoluene boiling at 83-84 C. is first obtained.

After removal of the water the constant boiling mixture toluene-formicacid distils over at 87 C.

which can be easily separated from the preced- 45 ing mixture by using acolumn. The base employed, which may be repeatedly used forconcentrating further quantities of dilute formic acid, remains behindin the distillation vessel.

The process is suitable for concentrating aqueous formic acid of anyconcentration. Pre-co-ncentration of the formic acid, e. g. to 77%, byordinary distillation, which is necessary in the case of many knownprocesses. is unnecessary. The bases to be employed for fixing theformic acid must boll at higher temperatures than the azeotropic mixtureof water and auxiliary liquid. In general, however, mixtures will beselected, which boil at higher temperatures than formic acid.Diethylaniline, quinoline and dimethylaniline have proved inter alia tobe very suitable.

In the process of this invention, however, bases, such as pyridine orpicoline, the use of which at first sight appears to be unsuitable, mayalso be employed. As Gardner (B. 23, page 1587, (1890)) has alreadydescribed, these bases on distillation with formic acid yieldazeo-tropes, which boil at substantially higher temperatures thanpyridine. These azeotropes have the composition of acid salts, possiblyof the composition 1 part of base and 3 parts of acid, or according toAndre (Comptes rendus 126, page 1205) 2 parts of base and 5 parts ofacid. Thus, if it is desired to decompose the neutral formates of thesebases thermally, the base is first distilled off, until the compositionof the acid salt is reached. This cannot be further decomposed by simpledistillation, so that the formic acid cannot be recovered in thismanner. These high-boiling mixtures can, however, be decomposedaccording to the invention. If, for example, toluene is added asauxiliary liquid to acid pyridine formate, the azeotrope formicacid-toluene distils over, until the composition of pyridine monoformateis reached. This may then serve for the formation of further acidformate.

As auxiliary liquid for the azeotropic distillation those liquids aresuitably selected, which are immiscible or only slightly miscible withformic acid and water in the cold, and are readily miscible with thebases employed. Hydrocarbons, e. g. toluene, or their halogenderivatives are inter alia suitable. The azeotropic mixtures, auxiliaryliquid-water and auxiliary liquid-formic acid respectively, areseparated during the distillation in separating vessels by layerformation and the auxiliary liquid is returned to the distillationcolumn. The counter-flowing vapours of the auxiliary liquid are washedthereby and in this way the efficiency of the columns is increased andthe carrying over of formic acid. by the mixture auxiliary liquid-waterand of the bases employed by the mixture auxiliary liquidformic acidavoided. No losses of the relatively expensive bases can accordinglyoccur.

The decomposition of the salt or salt mixture by azeotropic distillationproceeds already at relatively low temperatures, so that, for example,the strong acid only attacks the material of the vessel very slightly.In general the application of a vacuum is not necessary, the process,however, enables one to be used without difficulty in special cases. Theprocess of the invention can also be carried out continuously in asimple manner and without difficulties pertaining to apparatus.

The following examples illustrate how the process of the invention maybe carried into effect:

1. 200 kgms. of 35% formic acid are combined with 100 kgms. ofdiethylaniline (or quinoline) and distilled with the addition .of 100kgms. of toluene (or xylene) with the aid of an efficient column. Thetoluene-water mixture containing about 20% of water and distilling overat about 84 C. and the toluene-formic acid mixture containing of formicacid and distilling over at about 87 C. are separated into theircomponents in separators and the toluene is returned to the column.Almost the whole of the formic acid is obtained as a mixture of about90% formic acid and 10% toluene, from which the toluene can be easilyexpelled together with a little formic acid as an azeotropic mixture,which is returned to the apparatus, in a small auxiliary column. Afterseparating the toluene and the formic acid in themixture very highlyconcentrated formic acid is obtained.

2. 300 kgms. of 35% formic acid are added to kgms. of pyridine anddistilled with toluene. The operation is carried out as in Example 1 anda similar yield is obtained. Pyridine monoformate, which is employed forconcentrating further acid and is constantly regenerated, remainsbehind.

What I claim is:

1. A process for concentrating aqueous formic acid, which consists incombining the formic acid with an organic base to form a salt andazeotropically distilling ofi from this salt first the water with theaid of an auxiliary liquid and. then the formic acid with decompositionof the salt, said organic base boiling at higher temperature than theazeotropic mixtures of the auxiliary liquid with water and with formicacid.

2. A process according to claim 1, wherein the formic acid is combinedwith an organic base to form an acid salt.

3. A process according to claim 1, wherein the formic acid is combinedwith an organic base to form a neutral salt.

4. A process according to claim 1, wherein an auxiliary liquid isemployed, which is immiscible with formic acid and with water.

5. A process according to claim 1, wherein an auxiliary liquid isemployed, which is only slightly miscible with formic acid and withwater.

6. A process according to claim 1, wherein a substituted aniline isemployed as the organic base.

7. A process according to claim 1, wherein quinoline is employed as theorganic base.

8. A process for concentrating aqueous formic acid, which consists incombining the formic acid with an organic base, which, without anyauxiliary liquid, forms with formic acid acid salts boiling at highertemperatures than the base itself, and azeotropically distilling offfrom the resulting salt first the water with the aid of an auxiliaryliquid and. then the formic acid with decomposition of the salt, saidorganic base boiling at higher temperature than the azeotropic mixturesof the auxiliary liquid with Water and with formic acid.

9. A process according to claim 8, wherein pyridine is employed as theorganic base.

10. A process according to claim 8, wherein a monofo-rmate of the baseis employed.

' OTTO DRAGENDORFF.

